Semiconductor device with improved field plate

ABSTRACT

A transistor device includes a semiconductor body, a spacer layer, and a field plate. The spacer layer is over at least a portion of a surface of the semiconductor body. The field plate is over at least a portion of the spacer layer, and includes a first current carrying layer, a refractory metal interposer layer over the first current carrying layer, and a second current carrying layer over the refractory metal interposer layer. By including the refractory metal interposer layer between the first current carrying layer and the second current carrying layer, the electromigration of metals in the field plate is significantly reduced. Since electromigration of metals in the field plate is a common cause of transistor device failures, reducing the electromigration of metals in the field plate improves the reliability and lifetime of the transistor device.

FIELD OF THE DISCLOSURE

The present disclosure relates to transistor devices including fieldplates. Specifically, the present disclosure relates to transistordevices including field plates with reduced electromigrationcharacteristics.

BACKGROUND

Wide bandgap transistor devices are being used with increasing frequencydue to their performance advantages in high power, high temperature, andhigh frequency applications. While the advantages of wide bandgaptransistor devices are well known, wide bandgap materials systems havebeen slow to replace their conventional counterparts due to challengesencountered in the design and manufacture of wide bandgap devices. Onespecific challenge facing many wide bandgap transistor devices used inhigh frequency applications is reduced performance due to a capacitancebetween two or more electrodes of the device. For example, in the caseof field-effect devices and high electron mobility transistors (HEMTs),capacitive coupling between a gate electrode and a drain electrodereduces both the gain and breakdown voltage of the device.

In recent years, field plates have gained traction as a means formitigating the negative effects of undesirable capacitances betweenelectrodes of wide bandgap transistor devices. A field plate is aconductive plate that is placed over a portion of a charge transportlayer between two electrodes in the transistor device such that thefield plate is electrically isolated from the charge transport layer andthe electrodes. Generally, field plates are used in field-effect devicesand thus are placed between a gate and a drain electrode of the device,however, many different transistor devices may benefit from the use offield plates. The field plate reduces the capacitance between the gateand drain electrodes and redistributes an electric field on the drainside of the device in order to improve the break down voltage, gain, andmaximum operating frequency of the device.

While field plates have allowed wide bandgap transistor devices toachieve performance metrics that were previously unheard of, many fieldplates continue to suffer from relatively high electromigration of oneor more conductive materials within the field plate. Electromigration isthe movement or transport of material due to a transfer of momentumbetween conducting electrons and one or more atoms in the migratingmaterial. In other words, electromigration is the physical movement of amaterial over time due to a current flowing through the material. In thecase of field plates, electromigration may cause undesirable connectionsbetween the field plate and an electrode or charge transport layer dueto the migration of conductive material from the field plate through oneor more spacer layers. Additionally, electromigration may causedisconnection of a desired connection between the field plate and one ormore electrodes or voltage sources due to the migration of conductivematerial away from the desired connection points. Accordingly,electromigration of the field plate may cause the field plate to ceasefunctioning, reduced performance of the transistor device, and evencomplete failure of the transistor device.

In light of the discussion above, there is a present need for fieldplates with reduced electromigration characteristics and methods formanufacturing the same.

SUMMARY

The present disclosure relates to transistor devices including fieldplates with reduced electromigration characteristics. In one embodiment,a transistor device includes a semiconductor body, a spacer layer, and afield plate. The spacer layer is over at least a portion of a surface ofthe semiconductor body. The field plate is over at least a portion ofthe spacer layer, and includes a first current carrying layer, arefractory metal interposer layer over the first current carrying layer,and a second current carrying layer over the refractory metal interposerlayer. By including the refractory metal interposer layer between thefirst current carrying layer and the second current carrying layer, theelectromigration of metals in the field plate is significantly reduced.Since electromigration of metals in the field plate is a common cause oftransistor device failures, reducing the electromigration of metals inthe field plate improves the reliability and lifetime of the transistordevice. Further, including the first current carrying layer and thesecond current carrying layer in the field plate preserves theconductive capabilities of the field plate such that the field plateenhances the performance of the transistor device.

In one embodiment, the field plate further includes a first adhesionlayer on the surface of the spacer layer opposite the channel layer, asecond adhesion layer between the first adhesion layer and the firstcurrent carrying layer, and a protective overlayer over the secondcurrent carrying layer. In various embodiments, the first currentcarrying layer may have a thickness between 2500 Å and 3500 Å, therefractory metal interposer layer may have a thickness between 200 Å and300 Å, the second current carrying layer may have a thickness between2500 Å and 3500 Å, the first adhesion layer may have a thickness between150 Å and 250 Å, the second adhesion layer may have a thickness between250 Å and 350 Å, and the protective overlayer may have a thicknessbetween 25 Å and 75 Å.

In one embodiment, the refractory metal interposer layer may be a layerof nickel. The refractory metal interposer layer may be deposited viaphysical vapor deposition in various embodiments. The first adhesionlayer may be titanium (Ti), the second adhesion layer may be platinum(Pt), the first current carrying layer may be gold (Au), the refractorymetal interlayer may be nickel (Ni), the second current carrying layermay be gold (Au), and the protective overlayer may be titanium (Ti).

In one embodiment, the transistor device also includes a sourceelectrode, a drain electrode, and a gate, each in electrical contactwith the semiconductor body, such that the gate is between the sourceelectrode and the drain electrode. The spacer layer may be over at leasta portion of the gate, and the field plate may be positioned on thespacer layer such that a portion of the field plate is over the gate.The field plate may also be electrically connected to the sourceelectrode.

In one embodiment, a method of forming a transistor device includes thesteps of providing a semiconductor body, providing a spacer layer overat least a portion of a surface of the semiconductor body, and providinga field plate over at least a portion of the spacer layer. Providing thefield plate may include the steps of providing a first current carryinglayer, providing a refractory metal layer over the first currentcarrying layer, and providing a second current carrying layer over therefractory metal interposer layer.

Those skilled in the art will appreciate the scope of the presentdisclosure and realize additional aspects thereof after reading thefollowing detailed description of the preferred embodiments inassociation with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The accompanying drawing figures incorporated in and forming a part ofthis specification illustrate several aspects of the disclosure, andtogether with the description serve to explain the principles of thedisclosure.

FIG. 1 illustrates a field effect transistor including a field plateaccording to one embodiment of the present disclosure.

FIG. 2 shows a plan view of the field effect transistor and field plateshown in FIG. 1 according to one embodiment of the present disclosure.

FIG. 3 illustrates a field effect transistor including a field plateaccording to an additional embodiment of the present disclosure.

FIG. 4 shows a plan view of the field effect transistor and field plateshown in FIG. 3 according to one embodiment of the present disclosure.

FIG. 5 illustrates a high electron mobility transistor including a fieldplate according to one embodiment of the present disclosure.

FIG. 6 shows a plan view of the high electron mobility transistor shownin FIG. 5 according to one embodiment of the present disclosure.

FIG. 7 illustrates a high electron mobility transistor including a fieldplate according to an additional embodiment of the present disclosure.

FIG. 8 shows a plan view of the high electron mobility transistor shownin FIG. 7 according to one embodiment of the present disclosure.

FIGS. 9A through 9E show details of the field plates shown in FIGS. 1through 8 according to various embodiments of the present disclosure.

FIG. 10 is a block diagram describing a process for manufacturing afield-effect transistor including a field plate according to oneembodiment of the present disclosure.

FIGS. 11A through 11D illustrate the process shown in FIG. 10 accordingto one embodiment of the present disclosure.

FIG. 12 is a block diagram describing a process for manufacturing afield plate according to one embodiment of the present disclosure.

FIGS. 13A through 13D illustrate the process shown in FIG. 12 accordingto one embodiment of the present disclosure.

DETAILED DESCRIPTION

The embodiments set forth below represent the necessary information toenable those skilled in the art to practice the embodiments andillustrate the best mode of practicing the embodiments. Upon reading thefollowing description in light of the accompanying drawing figures,those skilled in the art will understand the concepts of the disclosureand will recognize applications of these concepts not particularlyaddressed herein. It should be understood that these concepts andapplications fall within the scope of the disclosure and theaccompanying claims.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the present disclosure. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

It will be understood that when an element such as a layer, region, orsubstrate is referred to as being “on” or extending “onto” anotherelement, it can be directly on or extend directly onto the other elementor intervening elements may also be present. In contrast, when anelement is referred to as being “directly on” or extending “directlyonto” another element, there are no intervening elements present.Likewise, it will be understood that when an element such as a layer,region, or substrate is referred to as being “over” or extending “over”another element, it can be directly over or extend directly over theother element or intervening elements may also be present. In contrast,when an element is referred to as being “directly over” or extending“directly over” another element, there are no intervening elementspresent. It will also be understood that when an element is referred toas being “connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present.

Relative terms such as “below” or “above” or “upper” or “lower” or“horizontal” or “vertical” may be used herein to describe a relationshipof one element, layer, or region to another element, layer, or region asillustrated in the Figures. It will be understood that these terms andthose discussed above are intended to encompass different orientationsof the device in addition to the orientation depicted in the Figures.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the disclosure.As used herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises,”“comprising,” “includes,” and/or “including” when used herein specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure belongs. It willbe further understood that terms used herein should be interpreted ashaving a meaning that is consistent with their meaning in the context ofthis specification and the relevant art and will not be interpreted inan idealized or overly formal sense unless expressly so defined herein.

FIGS. 1 and 2 show a transistor device 10 including a field plate 12according to one embodiment of the present disclosure. Specifically,FIG. 1 shows an embodiment wherein the transistor device 10 is ametal-semiconductor field-effect transistor (MESFET) including asubstrate 14, a buffer layer 16 over the substrate 14, and a channellayer 18 over the buffer layer 16. Collectively, the substrate 14, thebuffer layer 16, the channel layer 18, and any additional semiconductorlayers of the transistor device 10 are referred to as a semiconductorbody 20. A source electrode 22 and a drain electrode 24 are eachembedded in the channel layer 18 on opposite lateral edges 26 of thetransistor device 10, and a gate 28 is in electrical contact with thechannel layer 18 between the source electrode 22 and the drain electrode24. A spacer layer 30 is over the gate 28 and the exposed portions ofthe channel layer 18 opposite the buffer layer 16 and the substrate 14.Although the spacer layer 30 is shown covering the entirety of the gate28 and the exposed portions of the channel layer 18 between the sourceelectrode 22 and the drain electrode 24, the spacer layer 30 may coverless area so long as the spacer layer 30 is sufficient to support andisolate the field plate 12 from the gate 28 and the channel layer 18.The field plate 12 is over the spacer layer 30 such that the field plate12 partially overlaps the gate 28 and the channel layer 18 near thedrain electrode 24 without electrically contacting either. In oneembodiment, the field plate 12 extends between 0.1 to 2 microns from theedge of the gate 28 towards the drain electrode 24. A passivation layer(not shown) may be provided over the source electrode 22, the drainelectrode 24, the spacer layer 30, any exposed portions of the channellayer 18 and/or gate 28, and the field plate 12.

The field plate 12 may include one or more conductive paths 32 to thesource electrode 22. Generally, the conductive paths 32 are also on topof the spacer layer 30 such that they are electrically isolated from thegate 28 and the channel layer 18. Different sizes and numbers ofconductive paths 32 may be used in various embodiments. However, as thecombined area of the conductive paths 32 increases, so does theparasitic capacitance between the structures. Accordingly, a balancebetween the desired conductivity of the conductive paths 32 and theparasitic capacitance thereof should be struck. The spacer layer 30 mayindividually run under each one of the conductive paths 32 or may beprovided underneath the entire area of the conductive paths 32. In oneembodiment, a separate conductive path 34 may be formed that runsoutside of the active region of the transistor device 10, as shown inFIG. 2. For example, when the spacer layer 30 is not present over theportion of the channel layer 18 between the source electrode 22 and thegate 28, the separate conductive path 34 may be provided as shown.

The substrate 14 may be between 200 microns and 700 microns thick, andmay be formed from any material suitable for supporting growth of adesired semiconductor material for the buffer layer 16 and the channellayer 18. The buffer layer 16 may be between 0.1 microns and 2 micronsthick, and may be formed from a wide bandgap material such as siliconcarbide (SiC), gallium nitride (GaN), gallium arsenide (GaAs), or thelike. The channel layer 18 may be between 25 nm and 1 micron thick, andmay additionally be formed from any suitable wide bandgap semiconductormaterial. In one embodiment, a nucleation layer (not shown) may beincluded between the substrate 14 and the buffer layer 16 in order toreduce any lattice mismatch between the two. The spacer layer 30 may bebetween 0.05 microns and 2.0 microns thick, and may be formed of anysuitable material. For example, the spacer layer 30 may be a dielectricmaterial such as silicon nitride (SiN), silicon oxide (SiO₂), silicon(Si), germanium (Ge), magnesium oxide (MgO_(x)), magnesium nitride(MgN_(x)), zinc oxide (ZnO), or the like. In other embodiments, thespacer layer 30 may be formed of alloys or layer sequences of any of theabove mentioned materials.

The source electrode 22 and the drain electrode 24 may be formed ofseveral different materials such as titanium, titanium alloys, aluminum,gold, nickel, platinum, chromium, tungsten, platinum, silicide, as wellas combinations and alloys of the above. The gate 28 may similarly beformed of several different metals or alloys, and may have a gate lengthL_(G) between 0.3 microns and 0.8 microns. As discussed above, manytransistor devices including field plates suffer from reliability issuesdue to electromigration of metals in the field plate. Accordingly, thefield plate 12 includes at least two current carrying layers that areseparated by a refractory metal interposer layer, as discussed infurther detail below. The refractory metal interposer layer effectivelyreduces the electromigration potential of the field plate 12, therebypreventing movement of metals in the field plate 12 over time and thusincreasing the reliability of the transistor device 10. In variousembodiments, the same metal structure used for the field plate 12 issimilarly used for the source electrode 22, the drain electrode 24,and/or the gate 28 in order to reduce the electromigrationcharacteristics thereof. However, decreasing the electromigrationpotential generally comes at the expense of added resistance, andtherefore may not be suitable for the source electrode 22, the drainelectrode 24, and/or the gate 28 in all circumstances.

In operation, a first biasing voltage is applied between the sourceelectrode 22 and the drain electrode 24, and a second biasing voltage isapplied between the source electrode 22 and the gate 28. The respectivevalues of these biasing voltages determine the amount of current allowedto flow between the source electrode 22 and the drain electrode 24 ofthe device by varying the electric field present in the channel layer18. As discussed above, a capacitance present between the drainelectrode 24 and the gate 28 may produce undesirable effects in thetransistor device 10. Often, the result is a more pronounced electricfield at the drain side of the gate 28. Accordingly, the field plate 12is provided to reduce the gate-to-drain capacitance and the electricfield at the drain side of the gate 28. As a result, the transistordevice 10 may operate at higher voltages and frequencies than itsconventional counterparts.

FIGS. 3 and 4 show an additional embodiment wherein the transistordevice 10 is a MESFET. The transistor device 10 shown in FIGS. 3 and 4is substantially similar to that shown in FIGS. 1 and 2, except that thefield plate 12 shown in FIGS. 3 and 4 does not overlap the gate 28, butrather is separated from the gate by some distance between the gate 28and the drain electrode 24. Separating the field plate 12 from the gate28 as shown may prevent an increase in the input capacitance (gatecapacitance) of the transistor device 10 that may occur in theembodiment shown in FIGS. 1 and 2, while simultaneously reducing thecapacitance between the gate 28 and the drain electrode 24. Accordingly,the field plate 12 shown in FIGS. 3 and 4 may be desirable in someapplications wherein the input capacitance is an important designparameter.

FIGS. 5 and 6 show an embodiment wherein the transistor device 10 is ahigh electron mobility transistor (HEMT). In the embodiment shown inFIGS. 5 and 6, the semiconductor body 20 of the transistor device 10includes a substrate 36, a nucleation layer 38 over the substrate 36, abuffer layer 40 over the nucleation layer 38, and a barrier layer 42over the buffer layer 40. While not shown, additional layers may beincluded in the semiconductor body 20 of the transistor device 10without departing from the principles described herein. The sourceelectrode 22 and the drain electrode 24 may be embedded in the barrierlayer 42 such that the source electrode 22 and the drain electrode 24are in direct contact with the buffer layer 40. The gate 28 may be ontop of the barrier layer 42 such that the barrier layer 42 separates thegate 28 from the buffer layer 40. The spacer layer 30 is over the gate28 and the exposed portions of the semiconductor body 20 opposite thesemiconductor body 20. Although the spacer layer 30 is shown coveringthe entirety of the gate 28 and the exposed portions of thesemiconductor body 20 between the source electrode 22 and the drainelectrode 24, the spacer layer 30 may cover less area so long as thespacer layer 30 is sufficient to support and isolate the field plate 12from the gate 28 and the semiconductor body 20. In one embodiment, thefield plate 12 extends between 0.1 to 2 microns from the edge of thegate 28 towards the drain electrode 24. A passivation layer (not shown)may be provided over the source electrode 22, the drain electrode 24,the spacer layer 30, any exposed portions of the semiconductor body 20and/or gate 28, and the field plate 12.

The substrate 36 may be between 200 microns and 700 microns thick, andmay be formed from any suitable material suitable for supporting growthof a desired semiconductor material for the nucleation layer 38, thebuffer layer 40, and the barrier layer 42. The nucleation layer 38 maybe between 500 Å and 1500 Å thick, and may be formed from aluminumgallium nitride (AlGaN). The buffer layer 40 may be between 0.5 μm to 3μm thick, and may be formed from any Group III nitride material such asindium gallium nitride (InGaN), aluminum gallium nitride (AlGaN),aluminum nitride (AlN), or the like. In one embodiment, the buffer layer40 is a 2 μm thick layer of gallium nitride (GaN) with part of the layerdoped with iron (Fe). The barrier layer 42 may be between 50 Å and 400 Åthick, and may similarly be formed from a doped or undoped Group IIInitride material. In one embodiment, the barrier layer 42 comprises 0.8nm of aluminum nitride (AlN) and 22.5 nm of aluminum gallium nitride(AlGaN).

In operation, a first biasing voltage is applied between the sourceelectrode 22 and the drain electrode 24, and second biasing voltage isapplied between the source electrode 22 and the gate 28. The respectivevalues of these biasing voltages determine the electron concentration ina two-dimensional electron gas formed between the heterojunction of thebuffer layer 40 and the barrier layer 42. The higher the concentrationof electrons in the two-dimensional electron gas, the lower theresistivity thereof and thus the more current that is allowed to flowbetween the source electrode 22 and the drain electrode 24. As discussedabove, a capacitance present between the drain electrode 24 and the gate28 may produce undesirable effects in the transistor device 10,resulting in a higher resistivity and thus lower source-to-drain currentthan would otherwise occur. Accordingly, the field plate 12 is providedto reduce the gate-to-drain capacitance and the electric field at thedrain side of the gate 28. As a result, the transistor device 10 mayoperate at higher voltages and frequencies than its conventionalcounterparts.

FIGS. 7 and 8 show an additional embodiment wherein the transistordevice 10 is a HEMT. The transistor device shown in FIGS. 7 and 8 issubstantially similar to that shown in FIGS. 5 and 6, except that thefield plate 12 shown in FIGS. 7 and 8 does not overlap the gate 28, butrather is separated from the gate by some distance between the gate 28and the drain electrode 24. Separating the field plate 12 from the gate28 as shown may prevent an increase in the input capacitance (gatecapacitance) of the transistor device 10 that may occur in theembodiment shown in FIGS. 5 and 6, while simultaneously reducing thecapacitance between the gate 28 and the drain electrode 24. Accordingly,the field plate 12 shown in FIGS. 7 and 8 may be desirable in someapplications wherein the input capacitance is an important designparameter.

FIG. 9A shows details of the various layers of the field plate 12according to one embodiment of the present disclosure. For context, thespacer layer 30 is also shown. The field plate 12 includes a firstadhesion layer 44 on the surface of the spacer layer 30 opposite thechannel layer 18, a second adhesion layer 46 over the first adhesionlayer 44, a first current carrying layer 48 over the second adhesionlayer 46, a refractory metal interposer layer 50 over the first currentcarrying layer 48, a second current carrying layer 52 over therefractory metal interposer layer 50, and a protective overlayer 54 overthe second current carrying layer 52. The first adhesion layer 44 mayhave a thickness T_(FA) between 150 Å and 250 Å. The second adhesionlayer 46 may have a thickness T_(SA) between 250 Å and 350 Å. The firstcurrent carrying layer 48 may have a thickness T_(FC) between 2500 Å and3500 Å. The refractory metal interposer layer 50 may have a thicknessT_(I) between 200 Å and 300 Å. The second current carrying layer 52 mayhave a thickness T_(SC) between 2500 Å and 3500 Å. Finally, theprotective overlayer 54 may have a thickness T_(OL) between 25 Å and 75Å.

In one embodiment, the first adhesion layer 44 is titanium (Ti). Thesecond adhesion layer 46 may be platinum (Pt). The first currentcarrying layer 48 and the second current carrying layer 52 may be gold(Au). The protective overlayer 54 may be titanium (Ti) or platinum (Pt)in various embodiments. The refractory metal interposer layer 50 may beany refractory metal. For example, the refractory metal interposer layer50 may be nickel (Ni), cobalt (Co), titanium (Ti), or combinations ofmultiple elements such as titanium-tungsten-nitride (TiWN). As discussedherein, refractory metals are metals that are resistant to both heat andwear. Because the refractory metal interposer layer 50 is resistant toboth heat and wear, including the refractory metal interposer layer 50in the field plate 12 reduces the electromigration of metals in thefield plate 12. Accordingly, the reliability of the field plate 12 andthus the transistor device 10 is significantly improved. Specifically,by including the refractory metal interposer layer 50 in the field plate12, the likelihood of disconnection and/or shorting of the field plate12 is significantly reduced.

Although only a single refractory metal interposer layer 50 is shown inFIG. 9A, any number of refractory metal interposer layers 50 may be usedwithout departing from the principles of the present disclosure. Forexample, an additional refractory metal interposer layer 50 may beincluded between the second current carrying layer 52 and the protectiveoverlayer 54 in the field plate 12. While refractory metals aregenerally resistant to electromigration, such characteristics generallycome at the expense of increased resistance. That is, there is atrade-off between resilience and resistance that should be considered indesigning the field plate 12.

The primary purpose of the first adhesion layer 44, the second adhesionlayer 46, and the protective overlayer 54 is to protect the firstcurrent carrying layer 48, the refractory metal interposer layer 50, andthe second current carrying layer 52, as well as to provide adequatecontact for these layers to the outside environment. In variousembodiments, the first adhesion layer 44, the second adhesion layer 46,and the protective overlayer 54 may comprise any number of materialssuitable for performing this task.

The first adhesion layer 44, the second adhesion layer 46, the firstcurrent carrying layer 48, the refractory metal interposer layer 50, thesecond current carrying layer 52, and the protective overlayer 54 may bedeposited one on top of the other via any number of depositionprocesses. For example, the various layers may be deposited via asputtering process, an evaporation process, or the like. In oneembodiment, after the formation of some or all of the layers of thefield plate 12, an annealing process is performed in which the fieldplate 12 is heated. The annealing process may cause one or more of thevarious layers of the field plate 12 to dissolve into one another orcombine. The resulting combination of the layers is highly resistant toelectromigration and therefore preserves the reliability of thetransistor device 10 as discussed in detail above.

FIG. 9B shows details of the various layers of the field plate 12according to an additional embodiment of the present disclosure. Thefield plate 12 shown in FIG. 9B is substantially similar to that shownin FIG. 9A, but further includes an additional refractory metalinterposer layer 56 on top of the second current carrying layer 52 and athird current carrying layer 58 on top of the additional refractorymetal interposer layer 56. Further, the thicknesses of the variouslayers may differ in the embodiment shown in FIG. 9B. For example, thefirst adhesion layer 44 may have a thickness T_(FA) between 150 Å and250 Å, the second adhesion layer 46 may have a thickness T_(SA) between250 Å and 250 Å, the first current carrying layer 48 may have athickness T_(FC) between 4500 Å and 5500 Å, the refractory metalinterposer layer 50 may have a thickness T_(I) between 150 Å and 250 Å,the second current carrying layer 52 may have a thickness T_(SC) between4500 Å and 5500 Å, the additional refractory metal interposer layer 56may have a thickness T_(AI) between 200 Å and 300 Å, the third currentcarrying layer 58 may have a thickness T_(TC) between 4500 Å and 5500 Å,and the protective overlayer 54 may have a thickness T_(OL) between 25 Åand 100 Å. The additional refractory metal interposer layer 56 maycomprise the same materials as that of the refractory metal interposerlayer 50 discussed above. Further, the third current carrying layer 58may comprise any of the materials discussed above with respect to thefirst current carrying layer 48 and the second current carrying layer52.

FIG. 9C shows the various layers of the field plate 12 according to yetanother embodiment of the present disclosure. The field plate 12 shownin FIG. 9C includes a semiconductor layer 60 between a first refractorymetal interposer layer 62 and a second refractory metal interposer layer64. Further, the field plate 12 includes a first adhesion layer 66 ontop of the spacer layer 30, a third refractory metal interposer layer 68over the first adhesion layer 66, a second adhesion layer 70 over thethird refractory metal interposer layer 68 such that the second adhesionlayer 70 is between the first refractory metal interposer layer 62 andthe third refractory metal interposer layer 68, and a protectiveoverlayer 72 over the second refractory metal interposer layer 64. Thefirst adhesion layer 66 may have a thickness T_(FA) between 50 Å and 150Å, the third refractory metal interposer layer 68 may have a thicknessT_(TI) between 100 Å and 300 Å, the second adhesion layer 70 may have athickness T_(SA) between 75 Å and 150 Å, the first refractory metalinterposer layer 62 may have a thickness T_(FI) between 75 Å and 150 Å,the semiconductor layer 60 may have a thickness T_(S) between 350 Å and450 Å, the second refractory metal interposer layer 64 may have athickness T_(SI) between 75 Å and 150 Å, and the protective overlayer 72may have a thickness T_(OL) between 75 Å and 125 Å.

In one embodiment, the first adhesion layer 66 is titanium (Ti). Thesecond adhesion layer 70 may be platinum (Pt). The semiconductor layer60 may be silicion (Si). The protective overlayer 72 may be titanium(Ti) or platinum (Pt) in various embodiments. The first refractory metalinterposer layer 62, the second refractory metal interposer layer 64,and the third refractory metal interposer layer 68 may be nickel (Ni),cobalt (Co), titanium (Ti), or combinations of multiple elements such astitanium-tungsten-nitride (TiWN). Because the first refractory metalinterposer layer 62, the second refractory metal interposer layer 64,and the third refractory metal interposer layer 68 are resistant to bothheat and wear, as discussed above, including these layers in the fieldplate 12 reduces the electromigration of metals therein. Accordingly,the reliability of the field plate 12 and thus the transistor device 10is improved. Specifically, by including the first refractory metalinterposer layer 62, the second refractory metal interposer layer 64,and the third refractory metal interposer layer 68 in the field plate12, the likelihood of disconnection and/or shorting of the field plate12 is significantly reduced.

In one embodiment, the various layers shown in FIG. 9C show the fieldplate 12 before an annealing process in which the first refractory metalinterposer layer 62 and the second refractory metal interposer layer 64combine with the semiconductor layer 60. In the case that thesemiconductor layer 60 is silicon (Si), the resulting combination oflayers may be a silicide.

FIG. 9D shows the various layers of the field plate 12 according to yetanother embodiment of the present disclosure. The field plate 12 shownin FIG. 9D is substantially similar to that shown in FIG. 9C, exceptthat the field plate 12 shown in FIG. 9D does not include the thirdrefractory metal interposer layer 68 or the second adhesion layer 70.Accordingly, the first refractory metal interposer layer 62 sitsdirectly on top of the first adhesion layer 66. Further, the thicknessesof the various layers shown in FIG. 9D may differ from those discussedabove with respect to FIG. 9C. For example, the first adhesion layer 66may have a thickness T_(FA) between 25 Å and 75 Å, the first refractorymetal interposer layer 62 may have a thickness T_(FI) between 50 Å and150 Å, the semiconductor layer 60 may have a thickness T_(S) between 350Å and 450 Å, the second refractory metal interposer layer 64 may have athickness T_(SI) between 150 Å and 250 Å, and the protective overlayer72 may have a thickness T_(OL) between 25 Å and 75 Å.

In one embodiment, the various layers shown in FIG. 9C show the fieldplate 12 before an annealing process in which the first refractory metalinterposer layer 62 and the second refractory metal interposer layer 64combine with the semiconductor layer 60. In the case that thesemiconductor layer 60 is silicon (Si), the resulting combination oflayers may be a silicide.

Although the various embodiments are discussed above with respect toMESFETs and HEMTs, the disclosure is not so limited. That is, theprinciples of the present disclosure may be applied to any transistordevices in which a field plate is used. For example, the principles ofthe present disclosure may be applied to FET devices, metal-oxidesemiconductor FET (MOSFET) devices, and the like. Further, while theprinciples of the present disclosure are primarily directed towardsfield plates, they may equally be applied to other conductive portionsof transistor devices prone to electromigration such as electrodes,conductive traces, and the like.

FIG. 9E shows the various layers of the field plate 12 according to yetanother embodiment of the present disclosure. The field plate 12 shownin FIG. 9E is substantially similar to that shown in FIG. 9D, exceptthat the first refractory metal interposer layer 62, the secondrefractory metal interposer layer 64, and the semiconductor layer 60 arecombined into a single electromigration resistant layer 73, for example,by an annealing process. As discussed above, the semiconductor layer 60may be silicon (Si). Accordingly, the electromigration resistant layer73 may be a silicide layer. A thickness T_(ER) of the electromigrationresistant layer 73 may be between 500 Å and 1500 Å. In otherembodiments, the first refractory metal interposer layer 62 and thesecond refractory metal interposer layer 64 may only partially combinewith the semiconductor layer 60 such that the layers are still distinct.

FIGS. 10 and 11A through 11D illustrate a generalized process formanufacturing the transistor device 10 according to one embodiment ofthe present disclosure. First, the semiconductor body 20 is provided(step 100 and FIG. 11A). As discussed above, the semiconductor body 20may include a number of different layers, each comprised of differentmaterials with different doping levels. Providing each one of thedifferent layers in the semiconductor body 20 may be accomplished viaany suitable deposition or growth process. In some embodiments,providing a particular layer of the semiconductor body 20 may includemultiple deposition or growth steps.

The source electrode 22, the drain electrode 24, and the gate 28 arethen provided in electrical contact with one of the layers in thesemiconductor body 20, such that the gate 28 is between the sourceelectrode 22 and the drain electrode 24 (step 102 and FIG. 11B). In oneembodiment, a surface of the semiconductor body 20 is first etched toprovide a number of recesses in which the source electrode, the drainelectrode 24, and the gate 28 are respectively provided. Providing thesource electrode 22, the drain electrode 24, and the gate 28 may includedepositing the source electrode 22, the drain electrode 24, and the gate28 via any suitable deposition process. For example, the sourceelectrode 22, the drain electrode 24, and the gate 28 may be providedvia a sputtering process, an evaporation process, or the like. Further,providing the source electrode 22, the drain electrode 24, and the gate28 may include separately providing a number of different layers one ontop of the other to form the resulting contacts. A mask (not shown) maybe applied to appropriately control where the source electrode 22, thedrain electrode 24, and the gate 28 are provided in some embodiments.Upon depositing the source electrode 22, the drain electrode 24, and/orthe gate 28, the mask (not shown) may then be removed to expose theportions of the semiconductor body 20 between the source electrode 22,the drain electrode 24, and the gate 28.

Next, the spacer layer 30 is provided over the exposed portions of thesemiconductor body 20 and the gate 28 (step 104 and FIG. 11C). Thespacer layer 30 may be provided by either an epitaxial growth process ora deposition process. In some embodiments, the spacer layer 30 isprovided before the source electrode 22, the drain electrode 24, and thegate 28, and then etched along with the semiconductor body 20 in orderto form the recesses in which the source electrode 22, the drainelectrode 24, and the gate 28 are provided. A mask (not shown) mayadditionally be used to apply the spacer layer 30 to only the necessaryportions of the semiconductor body 20 and/or gate 28. For example, thespacer layer 30 may only be provided on the portion of the semiconductorbody 20 and/or gate 28 on which the field plate 12 will overlay.

Finally, the field plate 12 is provided over the spacer layer 30 (step106 and FIG. 11D). Generally, the field plate is provided by a suitabledeposition process such as a sputtering process, an evaporation process,or the like. Providing the field plate 12 may include separatelydepositing a number of different layers, such as the various layersdiscussed above with respect to FIG. 9. Further, providing the fieldplate 12 may include the use of a mask (not shown) to direct the fieldplate 12 to only a desired portion of the spacer layer 30. When thefield plate 12 is provided, the mask (not shown) may then be removed,leaving the device shown in FIG. 11D. In one embodiment, providing thefield plate includes an annealing step in which the field plate 12 (andpossibly the rest of the transistor device 10) is heated. As discussedabove, annealing the field plate 12 may cause mixing or dissolving ofone or more layers in the field plate 12, which may lead to a furtherreduction in the electromigration characteristics thereof. In oneembodiment, a passivation layer (not shown) is provided over the sourceelectrode 22, the drain electrode 24, the gate 28, any exposed portionsof the semiconductor body 20 and/or the spacer layer 30, and the fieldplate 12 in order to protect the transistor device 10.

FIGS. 12 and 13A through 13D show details of providing the field plate12 according to one embodiment of the present disclosure. First, a mask74 is provided over the source electrode 22, the drain electrode 24, andany exposed portions of the gate 28, the semiconductor body 20, and thespacer layer 30 (step 200 and FIG. 13A). The mask 74 may bepre-patterned with an opening 76 to expose a desired area of the spacerlayer 30 on which the field plate 12 is to be located. In otherembodiments, the mask 74 may be provided as a blanket layer and thenetched to form the opening 76. Generally, the opening 76 correspondswith the desired location of the field plate 12 on the spacer layer 30.

Next, the field plate 12 is provided in the opening 76 of the mask 74(step 202 and FIG. 13B). Providing the field plate 12 may includeproviding the field plate via a suitable deposition process such as asputtering process or an evaporation process. In one embodiment, eachlayer of the field plate is separately deposited one over the other inthe opening 76. The mask 74 is then removed (step 204 and FIG. 13C). Themask 74 may be removed by any suitable process. For example, the mask 74may be removed by a mechanical polishing process and/or a chemicaletching process. Finally, an optional annealing process is performed onthe field plate 12 (step 206 and FIG. 13D). In the annealing process,the field plate 12 is exposed to heat (represented by the wavy lines inFIG. 13D). In one embodiment, the field plate 12 is exposed to atemperature of 225° C. for a period of time between 5 and 10 minutes,however, any suitable annealing process may be used without departingfrom the principles of the present disclosure. The annealing process maycause one or more of the various layers within the field plate 12 todissolve into one another or otherwise combine. For example, the firstrefractory metal interposer layer 62 and the second refractory metalinterposer layer 64 may combine with the semiconductor layer 60 shown inFIGS. 9C and 9D to form a single electromigration resistant layer 63 asshown in FIG. 9E. In one embodiment, the resulting electromigrationresistant layer 63 is a silicide. Due to the various layers of the fieldplate 12 discussed above, the resulting field plate 12 is resistant toelectromigration and therefore much more resilient than its conventionalcounterparts. Accordingly, the reliability of the resulting transistordevice 10 is improved.

Those skilled in the art will recognize improvements and modificationsto the preferred embodiments of the present disclosure. All suchimprovements and modifications are considered within the scope of theconcepts disclosed herein and the claims that follow.

What is claimed is:
 1. A transistor device comprising: a semiconductorbody; a spacer layer over at least a portion of the semiconductor body;and a field plate over at least a portion of the spacer layer, whereinthe field plate comprises: a first current carrying layer; a refractorymetal interposer layer over the first current carrying layer; a secondcurrent carrying layer over the refractory metal interposer layer, afirst adhesion layer over the spacer layer opposite the semiconductorbody; a second adhesion layer between the first adhesion layer and thefirst current carrying layer; and a protective overlayer over the secondcurrent carrying layer, wherein: the first adhesion layer comprisestitanium; the second adhesion layer comprises platinum; the firstcurrent carrying layer and the second current carrying layer comprisegold; the refractory metal interposer layer comprises nickel; and theprotective overlayer comprises titanium.
 2. The transistor device ofclaim 1 wherein: the first current carrying layer has a thicknessbetween 2500 Å and 3500 Å; the refractory metal interposer layer has athickness between 200 Å and 300 Å; and the second current carrying layerhas a thickness between 2500 Å and 3500 Å.
 3. The transistor device ofclaim 2 wherein the refractory metal interposer layer comprises nickel.4. The transistor device of claim 1 wherein the refractory metalinterposer layer comprises nickel.
 5. The transistor device of claim 1wherein: the first current carrying layer has a thickness between 2500 Åand 3500 Å; the refractory metal interposer layer has a thicknessbetween 200 Å and 300Å; the second current carrying layer has athickness between 2500 Å and 3500 Å; the first adhesion layer has athickness between 150 Å and 250 Å; the second adhesion layer has athickness between 250 Å and 350 Å; and the protective overlayer has athickness between 25 Å and 75 Å.
 6. The transistor device of claim 1wherein the refractory metal interposer layer is deposited by physicalvapor deposition.
 7. The transistor device of claim 1 furthercomprising: a source electrode and a drain electrode in electricalcontact with the semiconductor body; and a gate in electrical contactwith the semiconductor body and between the source electrode and thedrain electrode.
 8. The transistor device of claim 7 wherein: the spacerlayer is over at least a portion of the gate; and the field plate ispartially positioned over the spacer layer opposite the gate such that aportion of the field plate is over the gate.
 9. The transistor device ofclaim 7 wherein the field plate is electrically connected to the sourceelectrode.
 10. The transistor device of claim 1 wherein at least two ofthe layers of the field plate are dissolved into one another.
 11. Amethod of forming a transistor device comprising: providing asemiconductor body; providing a spacer layer over at least a portion ofa surface of the semiconductor body; and providing a field plate over atleast a portion of the spacer layer, wherein providing the field platecomprises: providing a first current carrying layer; providing arefractory metal interposer layer over the first current carrying layer;and providing a second current carrying layer over the refractory metalinterposer layer; wherein the first current carrying layer and thesecond current carrying layer comprise gold; wherein the first currentcarrying layer has a thickness between 2500 Å and 3500 Å; the refractorymetal interposer layer has a thickness between 200 Å and 300 Å; and thesecond current carrying layer has a thickness between 2500 Å and 3500 Å.12. The method of claim 11 wherein the refractory metal interposer layercomprises nickel.
 13. The method of claim 11 wherein providing the fieldplate further comprises: providing a first adhesion layer over thespacer layer opposite the semiconductor body; providing a secondadhesion layer over the first adhesion layer such that the first currentcarrying layer is over the second adhesion layer; and providing aprotective overlayer over the second current carrying layer.
 14. Themethod of claim 11 wherein providing the field plate comprises providingthe field plate by one of a sputtering or evaporation process.
 15. Atransistor device comprising: a substrate; a nucleation layer over thesubstrate; a buffer layer over the nucleation layer; a barrier layerover the buffer layer; a spacer layer over at least a portion of thebarrier layer; and a field plate over at least a portion of the spacerlayer, wherein the field plate comprises: a first current carryinglayer; a refractory metal interposer layer over the first currentcarrying layer; and a second current carrying layer over the refractorymetal interposer layer; wherein the field plate further comprises: afirst adhesion layer over the spacer layer opposite the semiconductorbody; and a second adhesion layer between the first adhesion layer andthe first current carrying layer wherein the field plate furthercomprises a titanium protective overlayer over the second currentcarrying layer, and wherein the first adhesion layer comprises titanium;the second adhesion layer comprises platinum; the first current carryinglayer and the second current carrying layer comprise gold; and therefractory metal interposer layer comprises nickel.
 16. The transistordevice of claim 15 wherein the transistor device is a gallium nitride(GaN) high electron mobility transistor (HEMT).
 17. The transistordevice of claim 15 wherein at least two of the layers of the field plateare dissolved into one another.